Spark plug



R. TWELLS Nov. 4, 1952 SPARK PLUG Filed Deel. 30, 1946 INVENTOR To Soulac: or

Am Paesane:

s E M S maw E T WA T/ NVM E B o R4 mf Vl. B

Patented Nov. 4, 1952 UNITED STATES PATENT OFFICE SPARK PLUG Robert Twclls, Fostoria, Ohio Application December 30, 1946, Serial No. 719,303

1 Claim. l

This invention relates to spark plugs and to the method of making same, more particularly to the method of holding ceramic insulating tubes in a spark plug of the aviation type used in connection with a shielded ignition system.

In spark plugs used in connection with shielded ignition systems, such as used in aviation work, a ceramic tube is utilized in 1an upper barrel-like extension of the spark plug in which, at an intermediate joint, connection is made with an end of an insulated ignition cable and the terminal end of the center electrode, whereby the ignition potential may be led to the spark gap of the plug without substantial losses, such as from flash-over and the like. In the past, these tubes have been held in position by the use of springlike centering washers which have been undesirable, due to the fact that adverse tolerance in the cooperating parts place heavy stresses in the ceramic tubes which often resulted in breakage in the relatively frangible tubes. This breakage would some times occur at the time of manufacture and cause a large number of rejects of finished plugs, or, again, it would occur at a later time when the plug was in use in a spark plug engine. In the latter event, the breakage of the ceramic tube would cause dash-overs to destroy the usefulness of the plug.

The present invention contemplates a novel construction which will obviate this dificulty by introducing a method of holding the ceramic tube in position in the spark plug without any substantial strains being present therein and thereby avoid these destructive breakages and accompanying lash-overs. The invention further contemplates a method whereby this result can be eiliciently attained in a spark plug being suitable for use in mass production.

It is, therefore, a principal object of this invention to position a frangible ceramic insulating tube in ythe rbarrel of a spark plug of the shielded type so as to be free from stress and strain and yet resiliently held in position.

It is a further object of this invention to provide a method of positioning a ceramic tube concentrically within a metallic tube by a layer of stable, resilient organo-silicon oxide polymers between, whereby the ceramic tube is substantially free of stress and strain.

It is a further object of this invention to provide a device for conveniently positioning a layer of rubber-like silicone between concentric metal and ceramic tubes.

Other objects and advantages of this invention 4relating to the arrangement, operation and function of the related elements of the structure, to various details of construction, to combinations of parts and to economies of manufacture, will be apparent to those skilled in the art upon consderation of the following description and appended claim, reference being had to the accompanying drawings forming a part of this specication wherein like reference characters designate corresponding parts in the several views.

Referring to the drawings:

Fig. 1 is an elevational view, partly in section, of a spark plug incorporating the invention;

Fig. 2 is an enlarged elevational view, in section, of the shielding tube of the spark plug;

Fig. 3 is a plan view, in section, taken along the line 3 3 of Fig. 2;

Fig. 4 is an elevational view, in section, of a device for carrying out the invention.

Referring to the drawings, particularly to Fig. l, a spark plug is shown having a body member I0, with a threaded portion II to fit into an engine, and an external shoulder I2 adapted to cooperate with a sealing gasket (not shown). An internal shoulder I3 is shown, against which is seated an enlarged portion of an insulator I4 carrying a center electrode I5 provided with a firing tip I6 cooperating with a ground electrode II fitted into the body member I0. A gasket I8 is provided for sealing the insulator I4 in the shell I0 as shown.

To maintain a sealing pressure on the gasket I8, a spring member I3 is provided in the nature of a ring which bears on the upper shoulder of the enlarged portion of the insulator Ill. Tension is maintained on the spring member IQ, by a washer |911 thrust downwardly by the bottom end 20 of a shielding tube 22 threaded into the upper end of the body member I0 by a threaded section 23. The terminal end 24 of the center electrode I5 projects into the shielding tube 22 and terminates adjacent a central position thereof where contact is made with a conductor (not shown) adapted to project into the shielding tube from the opposite end and held in position by an attaching means (not shown) cooperating in threaded relation with threads 26 on the outside of the shielding tube 22.

To insulate the connection between terminal end 24 of the center electrode and the conductor from the metallic shielding tube 22, a ceramic tube 28 is provided which is tted into the interior of the shielding tube from the bottom end thereof and abuts against an inwardly extending flange 30 with a gasket 32, made preferably of asbestos therebetween at the upper end of the shielding tube. The dimension of the cooperating tubes are such that an annular space 34 is provided between them as shown. At the lower end of the ceramic tube 28 a second gasket 3B is provided to rest upon the washer i811. The method of positioning this ceramic tube within the metallic shielding tube is the locus of the invention to be described in detail hereinafter.

It is clear from the already described structure that the ceramic tube 28 must be placed in position in the metallic shielding tube 22 before the shielding tube is threaded into operative relation with the shell I8. Preferably the assembly is made as a separate sub-assembly for the spark plug as is clearly shown in Fig. 2. In order to hold the ceramic tube 28 resiliently in the metallic shielding tube 22, the space 34 therebetween is lled with a rubber-like body 38 of organosilicon oxide polymers, known in the trade as silicones. These polymers are very stable to moisture and temperatures found in the spark plug when in useA and form an` excellent resilient body to protect the relatively frangibleceramic tube from breakage. This is accomplished by creating a cushioning effect' for blows inicted by rough usage and also allows positioning of `the ceramic tube without stress and strain in the tube itself. It is highly desirable to position the body of these vpolymers as -compactly as possible and without vesiculation, for which purpose a novel machine is provided, as shown in Fig. 4 and described in detail hereinafter. The aforementioned machine is also described and claimed in application, Serial No. '719,304 filed December 30, 1946.

The organo-silicon oxide polymers most satisfactory for the aforementioned purpose are relatively heavy viscous fluids at room temperature, and can be obtained commercially under the trade-name of Silastic SC-69-3 (lc392) made by Dow Corning Corporation of Midland, Michigan. These polymers are in the form of. a soft, dense, white paste which will Ispread easily and will adhere tenaciously to a clean, smooth surface. It is particularly advantageous to use ya ceramic surface such as found on a ceramic article as it comes from the vitrifying furnace. Clean surfaces on the articles to which these polymers are to adhere 4are essential.

Referring now to Fig. 4, a device is shown which is designed to facilitate the positioning of a body of the aforementioned polymers between the ceramic tube 28 and the metallic shielding tube 22. For this purpose, a container 58 is provided, preferably made of brass having a lcubicle form with a cover 52. and a bott-om 54 in parallel arrangement. The container is made air-tight for purposes to be described hereinafter, being in communication with. a source of air pressure through conduit 56 positioned in the cover 52. A bleeder conduit 58 is provided, controlled by a valve 60, also in communication with the container 58 through the cover 52. The valve 68 allows an operator to control the air pressure in the chamber as measured by a gauge 52.

The floor 54 of the container 5D is provided with a centrally-located aperture 84, which is counter-bored and threaded to allow a metal shielding tube 22 to be fitted therein by the threads positioned onA its lower end as shown in Fig. 2. The tube is inverted for this, so that the flanged end thereof depends from the floor, substantially as shown with the aperture 64 coinciding with the internal diameter of the tube.

The cover 52 is provided with an aligning threaded aperture 6E, into which is tted the threaded end 68 of a cylinder 18, which depends from the cover and terminates a short distance above the licor 54 as shown. The cylinder 'i8 is locked in adjusted position in the cover 52 by lock nut l2, and is provided at its lower terminus by an enlargement 14 to form a lower f-ace i8, to which is Vattached a rubber packing 'i8 by means of a ring plate 88 held to the face by screws 82 which pierce the face. The rubber packing lt is provided with a central aperture 84 in align- Iment with the bore of the cylinder 'E8 and the 'aperture in the ring plate 88, which is champfered to provide a sharp wiping edge adjacent its upper surface somewhat smaller in diameter than the diameter of the bore of the cylinder 78. The aperture in the ring plate 88 is rounded downwardly to give a smooth surface to allow flexing of the gasket member downwardly when in use as described hereinafter.

The boreV of the cylinder E8 is of a diameter to allow the ceramic tube 28 to t loosely therein to allow ready longitudinal movement through the bore to hold the tube 28 in alignment with spaced longitudinal juxtaposition to the tube 22. When positioned in the bore, the tube 28 rests upon the upper surface of the gasket i8, while its upper end is adapted to cooperate with a plunger 88 adapted to be moved manually in the bore of the cylinder to thrust the ceramic tube 28 downwardly into partial engagement or telescoping relation with the metallic` shielding tube 22, thereby deflecting the gasket 78 downwardly to forma seal with the exterior of the tube 28. The operator begins a cycle of operation in this manner with the parts in position inthe device as vshown in Fig. 4.

A body 88 of the organo-silicon oxide polymers is positioned in the container 58 as shown with an air space above. Inasmuch as the polymer is a soft, dense, white paste of relatively high viscosity, it does not ow -readily without the application of substantial air pressure on its upper surface. It, therefore, does n-ot flow of its own volition through a narrow gap or circumambient orifice 88 formed between the ring plate and the bottom 54 of the container but must be eX- truded or forced to do so by the air pressure aforesaid.

To position a ceramic tube within a metallic shielding tube, an operator proceeds as follows: With the parts of the device in the position shown in Fig. 4, the plunger 85 is thrust downwardly to move the ceramic tube through the aperture in the gasket 18 which distends the flexible material thereof downwardly to form a seal with the outside of the tube and seals the gap or orifice 98 until the tube enters a slight distance the metallic shielding tube 22 threaded into the bottom. At this point, the valve 68 in lthe bleeder conduit 58 is closed to build up an air pressure over the body 88 of the polymers in the container until a pressure of '70 pounds per square inch is attained. Then simultaneously, theL plunger 88 is slowly moved downwardly to advance and telescope the ceramic tube 28 into the metallic tube 22, vand the air pressure in the container opens the seal formed in the gap or orifice 98 to a sufflcient extent to deposit a thin layer of the polymers on the outer surface of the ceramic tube, which carries the polymers into the metallic tube to lll completely the space 38 therebetween. This continues until the ceramic tube is completely telescoped with the metallic tube and is seated against the gasket 32 adjacent the ange 30 of the metallic tube 22. The air pressure is then released, the plunger retracted, and the completed tube assembly removed from the aperture 64. The ysecond gasket 36 is then positioned in the assembly as shown and the whole is heattreated at a temperature of 600 F. for one hour, which brings the polymers to a soft rubbery condition. The heat treatment cures the polymers and causes adherence to a maximum degree to both -members with which it is in contact.

To begin a new cycle of operation, a second metallic tube is positioned in the aperture 64, and a second ceramic tube is dropped into the bore of the cylinder 'l0 and the plunger replaced to the starting position as shown in Fig. 4.

The soit, rubbery condition of the layer of polymers, after heat-treatment between the ceramic tube 28 and the metallic tube 22, protects the ceramic tube from lateral stresses and holds the tube resiliently in position. There is substantially no stress or strain in the ceramic tube, either during assembly or after complete assembly and in use as a spark plug. This substantially obviates breakage arising from this source.

It is to be understood that the above detailed description of the present invention is intended to disclose an embodiment thereof to those skilled in the art, but that the invention is not to be construed as limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of being practiced and carried out in various Ways without departing from the spirit of the invention. The language used in the specification relating to the operation and function of the elements of the invention is employed for purposes of description and not of limitation, and it is not intended to limit the scope of the following claim beyond the requirements of the prior art.

What is claimed and it is desired to secure by United States Letters Patent:

In a spark plug of the shielded type, a lower metallic shell member with a threaded portion for fitting the spark plug into a suitable opening in a combustion chamber, a main insulator Seated concentrically in the lower shell member having a center electrode sealed therein, said main insulator having an upper terminus above the shell member and provided with external sealing means cooperating with the shell member, an upper metallic shielding tube tted at its lower end into the lower shell member, a ceramic tube housed concentrically in the shielding tube over the main insulator which terminates substantially at the center of said ceramic tube, said shielding tube having an inwardly projecting flange at its upper end to overlie the ceramic tube to hold it in position therein, said ceramic tube cooperating with the main insulator at its other end, and an elastic body of organo-silicon polymers positioned between the metallic shielding tube and the ceramic tube to resiliently hold it in position therein.

ROBERT TWELLS.

REFERENCES CITED The following references are of record in the .file of this patent:

UNITED STATES PATENTS Number Name Date 513,892 Curtis et al. Jan. 30, 1894 562,354 Calahart June 16, 1896 642,538 Thoma Jan. 30, 1900 1,643,021 Luyties Sept. 20, 1927 1,906,829 Acly May 2, 1933 2,130,879 Dobke Sept. 20, 1938 2,183,623 Ross Dec. 19, 1939 2,258,218 Rochow Oct. 7, 1941 2,356,104 Tognola Aug. 15, 1944 2,389,597 Cipriani Nov. 27, 1945 2,445,777 Hahn July 27, 1948 2,453,048 Tognola Nov. 2, 1948 2,464,487 Chappell et al. Mar. 15, 1949 

